1. Field of the Invention
The present invention relates to a cathode-ray tube provided with an electron gun having convergence plates.
2. Description of the Prior Art
The electron gun of the cathode-ray tube of the TRINITRON (a tradename) type is a so-called "one gun three beam" single electron gun, which is characterized in that three electron beams which are emitted from its three cathodes are diverged after the beams cross at a point in the center of the principal lens and these three electron beams are then converged again on the screen by action of convergence means. The convergence means, specifically is constructed of four convergence plates which consist of a pair of inner convergence plates and a pair of outer convergence plates. The inner convergence plates are supplied with an anode voltage HV from a voltage supply means disposed on the tube wall at the funnel portion of the cathode-ray tube and the pair of outer convergence plates are supplied with an equal convergence voltage CV which is lower than the anode voltage. Adjustment of the convergence is performed by changing the D.C. voltage CV which is supplied to the outer convergence plates so that deflection of the R (red) and the B (blue) beams which are located at both sides of the G (green) beam may be changed.
The convergence is thus adjusted by changing the voltage CV which is supplied to the convergence plates. However, even if the convergence is adjusted so that the G beam and R beam converge at the center of the screen, sometimes the B beam will not converge at the center of the screen and so-called horizontal misconvergence is produced, as shown in FIG. 4 due to errors in assembly of the electron gun or the like. Conventionally, the horizontal misconvergence has been corrected by means such as a six-pole magnet 61a-61f as shown in FIG. 5. Although the convergence has thereby been achieved, still there has been such a problem that the form of the beam spots 62R, 62G, and 62B are distorted as shown in FIG. 6 due to the influence of the magnetic field. This problem becomes more severe as the resolution of the cathode-ray tubes have been enhanced.
Convergence is also achieved at peripheral portions of the screen by action of the deflection yoke (DY), but, in reality, asymmetric misconvergence sometimes occurs in which the B beam deviates in the horizontal direction when the R and G beams are converged as shown in FIG. 3 due to errors in assembly of the deflection yokes, positioning errors of the electron gun when it is enclosed in the neck portion, and installation errors of the deflection yokes when installed in the tube. So far, when the B and R beams are symmetrically deviated with respect to the G beam which is in between, adjustment has been achieved by dynamically applying to the outer convergence plates a common voltage having a parabolic waveform. However, in the prior art, it has not been possible to correct the misconvergence which occurs asymmetrically in the horizontal direction at both end portions on the X axis of the screen and at the four corners of the screen.